Mars may be slowly crushing Phobos into a planetary ring

By Graham Templeton 

Moons are weird. Sometimes they’re pulled out of a peaceful path through space, falling easily into orbit around a planet. Other times, they’re ripped out of the substance of the planet itself by an enormous collision, as a gargantuan planet-chunk slowly becomes spherical thanks to internal gravitational forces. Some moons are spinning slowly outward, destined to someday escape and fly off to explore the cosmos. Others and spiraling inward, and will someday crash to devastate their parent.

Phobos, the inner-most of Mars’ two moons, may have an even more unusual life history: Rather than spiraling inward to eventually collide with the planet, new evidence suggests that Phobos is being slowly crushed by the gravity of Mars, and that it will one day crumble to become the dusty components of a planetary ring. Phobos spirals in by a few inches per century, meaning that over millions of years, it will slowly be subject to a larger and larger proportion of Martian surface gravity.

Phobos, caught transiting the Sun.

Astronomers first observed the “stretch marks” on Phobos some time ago, but assumed they were cracks caused by a major meteor impact, or perhaps by many smaller ones. Now, new modeling suggests a different explanation: the grooves that can be seen on the side of Phobos are actually signs the moon is slowly being crushed. The evidence was presented at the Meeting of the Division of Planetary Sciences of the American Astronomical Society at National Harbor, Maryland.

The big new idea is that Phobos is a “rubble pile” moon at the center, a loose collection of rocks that have not fused to become a solid whole, and which thus deform easily beneath about 330 feet of regolith (Martian dirt). Phobos is closer to its parent planet than any other moon in the solar system (and thus closer than any other moon known, period) so those forces are quite strong. The regolith seems to act like a a sort of fabric, able to take some amount of stress in a slightly ductile way, but then failing under those stresses and cracking as seen on the surface.

Neptune has another potential rubble-pile moon that’s also slowly spiraling inward, and also shows outward signs of stress. Studying Phobos could shed light on a whole type of moon, and it could help to explain the formation of many planetary ring systems. A potential Phobos-born ring around Mars would likely only exist for between one and one hundred million years before the majority of the particles were pulled down into the surface of the planet; think about the volume of stuff that must have once been swirling around Saturn, for us to still be able to find so much of it in a relatively stable orbit.

As with all planetary science, this has implications for the study of exoplanets. Distant moons are currently too small to observe directly, but they do follow the same basic physical rules as all other bodies in the universe. How they break up while falling into their host planet could be broadly similar to how a planet breaks up while falling into its host star.